In general, the synchro separation circuit is constituted by an amplitude separation circuit for separating a composite synchronizing signal from a composite video signal and a frequency separation circuit separating a vertical synchronizing signal from the composite synchronizing signal.
Hitherto, in the amplitude separation circuit of the synchro separation circuit, if the separation sensitivity is increased when the composite synchronizing signal is desired to be separated from the composite video signal, an undesirable phenomenon so-called "video-in-sink" occurs wherein a video signal component of the composite video signal is separated as a synchronizing signal and, therefore, the separation sensitivity had hitherto been determined in reference to the separation of a horizontal synchronizing signal during the video signal duration.
However, since no video signal appear during the vertical blanking period, no video-in-sink occur and, therefore, it is possible to increase the separation sensitivity of the vertical synchronizing signal. Despite of this possibility, if the separation sensitivity of the amplitude separation circuit is set in reference to the separation of a horizontal synchronizing signal during the video signal duration, no vertical synchronizing signal can be separated without any fault when in a weak electric field, thereby posing a problem.
On the other hand, a frequency separation circuit generally utilizes an integrator to separate the vertical synchronizing signal. In this case, an integrating capacitor (and a resistor if necessary) has to be of a relatively large value and, therefore, where the synchro separation circuit is desired to be fabricated in an integrated circuit (hereinafter referred to as IC), the capacitor and the resistor have to be used as members separate from the IC. In addition, even though they can be fabricated in the IC, a large space is required for the installation of these component parts in the IC. Moreover, in the conventional circuit, the circuit itself is very complicated and hampers, in the presence of the above described disadvantages, the miniaturization and cost-reduction of the IC.
Accordingly, the present invention has been developed with a view of eliminating the above described disadvantages inherent in the conventional synchro separation circuit and has for its first object to provide a synchro separation circuit effective to assuredly separate the vertical synchronizing signal even when in a weak electric field, by increasing the selection sensitivity of the amplitude separation circuit during at least the vertical synchronizing signal period of the composite video signal.
A second object of the present invention is to provide a synchro separation circuit wherein an integrator for the composite synchronizing signal is constituted by connecting a Miller integrating capacitor to a current mirror circuit in order for a capacitor of small capacitance to be utilized, in achieving a large integrating effect, thereby enabling the circuit to be fabricated into an IC.
A third object of the present invention is to provide a synchro separation circuit wherein there is employed an input transistor of the amplitude separation circuit and a transistor constituting an emitter-follower path for an impedance conversion for supplying the composite video signal to this transistor, which are opposite in operating state to each other, so as to avoid any possible occurrence of the video-in-sink which sould result from the emitter current of the input transistor of the amplitude separation circuit.